1. Field of the Invention
This invention relates to an improved portland cement, and more specifically to a hydraulic portland cement containing a strength enhancing additive. The strength enhancing additive of this invention comprises triisopropanolamine, which may be added to cement clinker as a grinding aid and pack set inhibitor or may be added as an admixture to cement powder to enhance the 28-day strength of the mix.
2. Description of the Prior Art
The term cement is used to designate many different kinds of materials useful as binders or adhesives. Hydraulic cements are powdered materials which, when mixed with water, form a paste that hardens slowly to form rock-hard products such as mortar or concrete. Portland cement is distinguished from other cements by the different components of which it is composed, and the requirement that it meet particular standard specifications established in each country (see Cement Standards of the World, Cembureau, Paris, Fr.) For example, in the U.S., the American Society for Testing and Materials (ASTM), American Association of State Highway and Transportation Officials, as well as other government agencies, have set certain basic standards for cement which are based on principal chemical composition requirements of the clinker and principal physical property requirements of the final cement mix. For purposes of this invention the term portland cement is intended to include all cementitious compositions which meet either the requirements of the ASTM (as designated by ASTM Specification C150), or the established standards of other countries.
Portland cement is prepared by sintering a mixture of components including calcium carbonate (as limestone), aluminum silicate (as clay or shale), silicon dioxide (as sand), and miscellaneous iron oxides. During the sintering process, chemical reactions take place wherein hardened nodules, commonly called clinkers, are formed. Portland cement clinker is formed by the reaction of calcium oxide with acidic components to give, primarily tricalcium silicate dicalcium silicate, tricalcium aluminate, and a ferrite solid solution phase approximating tetracalcium aluminoferrite. The conventional cement chemists notation uses the following abbreviations:
CaO=C PA0 SiO.sub.2 =S PA0 Al.sub.2 O.sub.3 =A PA0 Fe.sub.2 O.sub.3 =F PA0 tricalcium silicate=C.sub.3 S PA0 dicalcium silicate=C.sub.2 S PA0 tricalcium aluminate=C.sub.3 A PA0 tetracalcium aluminoferrite=C.sub.4 AF
thus:
After the clinker has cooled, it is then pulverized together with a small amount of gypsum (calcium sulfate) in a finish grinding mill to provide a fine, homogeneous powdery product known as portland cement. Due to the extreme hardness of the clinkers, a large amount of energy is required to properly mill them into a suitable powder form. Energy requirements for finish grinding can vary from about 33 to 77 kW h/ton depending upon the nature of the clinker. Several materials such as glycols, alkanolamines, aromatic acetates, etc., have been shown to reduce the amount of energy required and thereby improve the efficiency of the grinding of the hard clinkers. These materials, commonly known as grinding aids, are processing additives which are introduced into the mill in small dosages and interground with the clinker to attain a uniform powdery mixture. In addition to reducing grinding energy, the commonly used processing additives listed above are frequently used to improve the ability of the powder to flow easily and reduce its tendency to form lumps during storage.
Various other additives may be incorporated into the cement to alter the physical properties of the final cement. For example, alkanolamines such as monoethanolamine, diethanolamine, triethanolamine and the like are known to shorten the set time as well as enhance the one-day compressive strength of cements. However, these additives usually have little beneficial effect on the 28-day strength of the finished cement, and in some cases may actually diminish it. Various other polymeric amines and imines have been used as 28-day cement strength enhancers, however, due to the high costs of these additives, their use is somewhat limited. In many countries, a portland cement may contain up to 5% fillers or clinker substitutes. As used herein, the term "filler" refers to an inert material that has no later age strength enhancing attributes; the term "clinker substitute" refers to a material that may contribute to long term compressive strength enhancement, but usually exhibits little or no enhancement of 7 or 28-day compressive strength values. The 28-day compressive strength has particular significance and will be emphasized throughout this invention since it is the strength at this age which is most commonly used to assess the engineering properties of the final cement products.
It has now been discovered that triisopropanolamine (TIPA), which was previously believed to possess the equivalent cement additive properties as triethanolamine (TEA) (i.e. shortening set times and enhancing one-day compressive strengths), surprisingly exhibits unique strength enhancing properties, namely a marked increase in the 7 and 28-day compressive strengths of certain hydraulic portland cements. The portland cements suitable for use in this invention are those containing at least 4% tetracalcium aluminqferrite (C.sub.4 AF) by weight. The enhanced 7 and 28-day strengths exhibited by these cements were unexpected and unobvious since C.sub.4 AF was believed to have no cementing value.